Treatment method for waste water sludge comprising phoshorous, heavy metals and at least one metal

ABSTRACT

Waste water sludge which contains phosphorous, heavy metals and at least one metal originating from a waste water treatment coagulant, is treated in order to recover the phosphorous and the at least one metal, and to discharge the heavy metals. The at least one metal includes iron. The treatment method includes the steps of acidifying the sludge to dissolve metals contained therein; subjecting the acidified sludge to liquid-liquid extraction to thereby provide an aqueous phase or raffinate, and recovering at least a portion of the at least one metal; treating the raffinate, during a first precipitation stage, with an effective amount of chemicals to provide a precipitate including heavy metals, and a solution having a decreased content of heavy metals; discharging the precipitate including heavy metals; treating the solution having a decreased content of heavy metals, in a second precipitation stage, with an effective amount of chemicals to provide a precipitate including phosphorous; and recovering phosphorous from the precipitate including phosphorous.

TECHNICAL FIELD

The invention relates to a method for treating waste water sludgecomprising at least one metal, especially iron and possibly aluminium,originating from a waste water treatment coagulant, and phosphorus andheavy metals in order to recover said at least one metal and phosphorusand to discharge said heavy metals. In particular, the sludge is from awaste water purification process where waste water is chemicallyprecipitated using coagulants containing iron and possibly aluminium.

BACKGROUND OF THE INVENTION

Disposal of waste water sludge is a major problem in water purificationplants. This is partly due to the heavy metal content of the sludge. Itis difficult to find suitable places for the waste and as standards riselandfilling is becoming more and more expensive. From this perspectivethe idea of recycling the waste water sludge is becoming increasinglyimportant. A complete recycling of waste water sludge would involverecycling of coagulants (iron and aluminium), part of the organicsubstances of the sludge, recovery of phosphorus, and separation of theheavy metals from the sludge. Until now recycling of the sludge has beenrealized only partially. There are no existing production methods forseparating coagulant chemicals and phosphorus from the sludge.

Sludge comes from various sources of the waste water purificationprocess i.e. from pre-precipitation, simultaneous precipitation andpost-precipitation stages. One possible treatment procedure for thesludge is first to dewater it to a dry solids content of 15-25% and thento use in agriculture, compost, incinerate or transport the dewateredsludge to a dump.

Another possible procedure is to acidify the precipitation sludge todissolve metals. Insoluble substances are removed by filtering. Thedissolved metals and phosphorus in the filtrate are precipitated and asludge, which will be called a metal sludge, is obtained. The metalsludge contains the iron and aluminium of the used coagulant and, inaddition, phosphorus and heavy metals. The procedure can also beperformed at an elevated temperature to improve yield and filterabilityi.e. the dewatering properties of the sludge. The sludge to be treatedcan be a pre-precipitation sludge, a simultaneous precipitation sludge,post-precipitation sludge or a mixture thereof.

One additional alternative for treating the sludge is hydrolysis wherethe purpose is to hydrolyse organic material of the sludge intoshort-chained compounds to be utilized in later stages of the wastewater treatment process, especially as carbon source in thedenitrification stage. During hydrolysis, the metals of the raw sludgedissolve in the hydrolysate solution. In the so-called thermal acidhydrolysis the temperature is 150°-160° C. and pH<2 preferably 1-1.5.After the hydrolysis, the insoluble part i.e. the organic sludge isseparated, the sludge containing mainly insoluble organic and partlyinorganic material e.g. fibres and silicate minerals. The pH of theobtained solution is raised above the neutral level using a base so thatthe dissolved metals precipitate as hydroxides and phosphates. Theprecipitated sludge i.e. the metal sludge is then separated. The metalsludge contains iron and aluminium and also phosphorus and heavy metals.

Acidification nor hydrolysis of sludge is not commonly used in wastewater purification. One reason is poor profitability. An additionalproblem is the metal sludge which has no use. The metal sludge containsheavy metals which makes the sludge a harmful waste for the environment.

The metal sludge can be dissolved in sulphuric acid or possibly inhydrochloric acid and the insoluble substances can be filtered. Theacidic filtrate solution contains the coagulants, phosphorus and heavymetals. It cannot be recycled or utilized in any other way as such andthere are no existing methods to separate the elements.

Solvent extraction i.e. liquid-liquid extraction is a well-known methodfor separating different elements from each other and, in principle, itcould be used to separate said elements. However, there are difficultiesin applying extraction to the acidified metal sludge of the above kindor to any other acidified waste water sludge. The acidic solutionobtained by leaching waste water sludge with sulphuric acid not onlycontains dissolved metals but also insoluble fine solid particles,colloidal components, humic acids etc. These impurities comprise anundesirable organic residue (crud) which has the most unfavourableeffect on extraction. It significantly retards mass transfer and phasesdisengagement. In disengagement of phases, after contacting the organicand aqueous phases, this substance usually collects as a separate layerbetween the phases. Therefore, the existence of the insoluble residuehas prevented extraction methods from being exploited in the recovery ofiron and aluminium from acidified sewage sludge.

Cornwell and Zoltek (J. Water Pollut. Control Fed., Vol 49, p. 600-612)have presented an extraction method for the recovery of aluminium from asludge containing aluminium as aluminiumhydroxide. The extractionsolvent is a mixture of monoethyl hexyl phosphate (MEHPA) anddi-(2-ethyl hexyl) phosphate (DEHPA). A process employing solventextraction with organic extractants for the removal of iron from aqueousacidic solutions has been suggested in the patent publication EP 58148.The objective of the method was to recover pure acid by extracting ironions into organic solvent. Solvent extraction used for the selectiverecovery of dissolved iron and aluminium can, with a proper solvent,efficiently separate iron and aluminium from heavy metals.

SUMMARY OF THE INVENTION

The objective of this invention is to provide a method whereby thesludge problem in a waste water purification plant can be solved. Themain objective is to provide a method whereby iron, aluminium andphosphorus of a metal sludge can be recovered. Iron and aluminium can berecycled in the purification process, phosphorus can be used e.g. as afertilizer and, at the same time, the sludge containing the heavy metalscan be separated and more easily disposed. This objective can beaccomplished by the present invention, and thus the present inventionprovides a method for treating waste water sludge comprising at leastone metal originating from a waste water treatment coagulant, andphosphorus and heavy metals in order to recover said at least one metaland phosphorus and to discharge said heavy metals, said methodcomprising:

acidifying said waste water sludge to dissolve metals contained in thesludge;

subjecting the acidified waste water sludge to liquid-liquid extractionfor recovering at least a portion of said at least one metal;

treating the raffinate of the extraction in a first precipitation stagewith appropriate chemicals for precipitating heavy metals, andthereafter discharging the precipitate thereby leaving a solution havinga decreased content of heavy metals; and

treating the solution having a decreased content of heavy metals in asecond precipitation stage with appropriate chemicals for precipitatingphosphorus and thereafter recovering the precipitated phosphorus.

Preferably the waste water sludge to be treated with the method of thepresent invention comprises metal sludge obtained by subjecting wastewater sludge from a waste water treatment plant to acid treatmentfollowed by precipitation of metal sludge from the filtrate.

DETAILED DESCRIPTION OF THE INVENTION

The method of the invention is a significant improvement compared to theconventional practice in the field and offers a sustainable solution forthe utilization of waste water sludge. The invention renders utilizationof the precipitated metal sludge possible. Phosphates and raw materialfor coagulant chemicals can be produced from the metal sludge.Furthermore, heavy metals can be separated whereby they do not harm theenvironment. The method improves profitability of sludge acidificationor hydrolysis. In the method of the invention, Fe and Al are recoveredas a solution which can be used in coagulation. Phosphorus isprecipitated as calcium phosphate or magnesium ammonium phosphate, forexample, which can be used in fertilizers.

Heavy metals of the metal sludge are separated by precipitation. Theamount of the precipitated sludge containing heavy metals is small sothat dumping of the sludge is easier and does not cause any harm toenvironment.

According to the invention the acidified sludge solution can be treatedwith an oxidizer like a strong solution of hydrogen peroxide prior tothe liquid-liquid extraction stage to transform the organic substancesof the sludge solution into such a form that does not have anunfavourable effect on the extraction and to oxidize the bivalent ironof the sludge solution into a trivalent iron. When hydrogen peroxide isadded into a sludge with bivalent iron and organic compounds, aso-called Fenton-reagent is formed. This means that bivalent iron andhydrogen peroxide form hydroxyl radicals which function as very strongoxidizer being able to tackle the organic compounds with the desiredeffect. Other alternative oxidizers are oxygen, ozone, potassiumpermanganate, solution of potassium dichromate, chlorine, and chlorinedioxide. After this treatment the solution can be filtered and thefiltrate solution is then taken to the liquid-liquid extraction stage.

According to the invention, it is also possible to remove organicmaterial so that the waste water sludge is calcined before acidificationat a temperature of 300° C. or over whereat organic material burns out.The calcination is to be performed in an atmosphere with an excess ofoxygen. In this case oxidation is not needed.

The liquid-liquid extraction stage comprises an extraction stage and astripping stage. In the extraction stage, said acidified waste watersludge is brought into contact with an extraction solution which isimmiscible with water for forming an aqueous phase i.e. raffinate and anorganic phase which organic phase contains ions of said metal. Theraffinate is separated from the organic phase. In the stripping stage,the organic phase is brought into contact with an acidic aqueousstripping solution for forming an aqueous phase and an organic phase.The aqueous phase containing the desired metal ions is separated fromthe organic phase.

The extraction solution contains an organic phosphate, an organicsolvent and possibly a long-chained alcohol. The organic phosphate isadvantageously an alkyl phosphate like a monoalkyl phosphate e.g. amono-(2-ethyl hexyl) phosphate (MEHPA), dialkyl phosphate e.gdi-(2-ethyl hexyl) phosphate (DEHPA) or trialkyl phosphate e.g. tributylphosphate, or a mixture thereof e.g. a mixture of MEHPA and DEHPA(MDEHPA). The organic solvent is advantageously a long-chainedhydrocarbon solvent like kerosene. The long-chained alcohol can be2-octanol, for example.

The stripping solution advantageously consists of hydrochloric acid orsulphuric acid. Stripping can also be performed in reducing conditionswhich can be accomplished by using a stripping solution containingsulphuric acid obtained by bubbling sulphur dioxide in water or dilutesulphuric acid.

According to the invention it is possible to subject the raffinate ofthe extraction prior to the first precipitation to a treatment with analkali hydroxide for precipitating a further portion of iron andpossibly aluminium as phosphates, and thereafter separating thephosphate precipitate. The alkali hydroxide, e.g. sodium hydroxide, ispreferably added in an amount to raise the pH to a value between 2 and4. The separated phosphate precipitate can be treated with an alkalihydroxide, e.g. sodium hydroxide, thereby forming insoluble ironhydroxide and a solution comprising soluble alkali phosphate andpossibly aluminium hydroxide. Said solution can be introduced into thefirst or second precipitation stage.

According to the invention, the acidic raffinate is treated in the firstprecipitation stage with a heavy metal binder, like hydrogen sulphide orsulphide, e.g. sodium sulphide or sodium hydrogen sulphide or ferroussulphide, at a suitable pH value for precipitating heavy metals and ironand at which pH aluminium can precipitate as a hydroxide or a phosphate.The precipitate is separated and the obtained aqueous solution istreated in a second precipitation stage with a reagent precipitatingphosphorus, like calcium or magnesium oxide, calcium or magnesiumhydroxide or calcium or magnesium chloride, at a suitable pH value forprecipitating phosphorus as phosphates, like calcium or magnesiumphosphate. In the presence of ammonia, the phosphorus can beprecipitated specifically as MgNH₄ PO₄ (MAP). This provides a means toremove ammonia from the solution. Part of the Mg used in thisprecipitation can also originate from the waste water sludge.

The method according to the invention can further comprise a thirdprecipitation stage wherein aluminium is precipitated as aluminiumhydroxide.

According to an advantageous embodiment the acidic raffinate is treatedin the first precipitation stage with sodium sulphide adjusting the pHto a value within 5-8, using sodium hydroxide, MgO or Mg(OH)₂, orammonia, for precipitating heavy metals and iron, which has come throughthe extraction stage, and possibly aluminium. The precipitate isseparated and the aqueous solution obtained is treated in the secondprecipitation stage with a calcium or magnesium compound like calciumchloride, magnesium oxide, magnesium hydroxide or magnesium chloridepossibly adjusting the pH value for precipitating calcium phosphate ormagnesium ammonium phosphate, and the precipitate obtained is separated.

According to another advantageous embodiment of the invention, theacidic raffinate is treated in the first precipitation stage with a S²⁻or HS⁻ containing reagent and, at the same time with the sulphideaddition, raising the pH to a value of about 6-7, using a base likesodium hydroxide, for precipitating heavy metals and iron and possiblyaluminium. Then the pH is adjusted to a value of over 9, preferablyabout 12 using sodium hydroxide, whereby the possibly precipitatedaluminium is redissolved. The precipitate containing heavy metals isthen separated. The aqueous solution containing phosphorus and aluminiumis treated in the second precipitation stage with calcium chloride forprecipitating calcium phosphate. The precipitate obtained is separated.The aqueous solution containing aluminium is treated in the thirdprecipitation stage with an acidic compound like e.g. sulphuric acid forprecipitating aluminium hydroxide and the precipitate obtained isseparated.

A modification to the above embodiment, which is particularly suitablefor removing possible Cr from the raffinate, is as follows. In the firstprecipitation stage, the pH of the acidic raffinate is adjusted to avalue of about 3-4, using sodium hydroxide, and then the raffinate istreated with sodium sulphide for precipitating some of the heavy metalsand iron residuals and possibly minor amounts of aluminium. Theprecipitate, which also contains Cr as chromium phosphate, is separated,after which pH is adjusted to a value of about 12 and over using sodiumhydroxide. The precipitate possibly formed is separated. Thisprecipitate contains also heavy metals. The aqueous solution containingAl and PO₄ is treated in the second precipitation stage with calciumchloride for precipitating calcium phosphate. The precipitate isseparated and the aqueous solution obtained is treated in the thirdprecipitation stage with sulphuric acid for precipitating aluminiumhydroxide and the precipitate obtained is separated. The twoprecipitates containing heavy metals are united for disposal.

The method of the invention partly solves the sludge problem in a wastewater purification plant by improving usability of the solid sludge ofthe waste water purification process. This is due to several factors.First, the amount of solid material decreases. There are less heavymetals in the sludge whereby it has better possibilities to be used inagricultural applications and incineration is safer. The calorific powerof the dry solids is higher and the amount of remaining ash is lower. Inaddition, the heavy metal content of the ash is lower. Since theprecipitated sludge which contains the heavy metals has a relativelysmall volume, it can be disposed of in a controlled manner without anyharm to the environment.

DESCRIPTION OF THE DRAWING

The invention is described in more details in the following referring tothe enclosed drawings in which

FIG. 1 shows an acidification process of precipitated sludge as a blockdiagram,

FIG. 2 shows a method according to the invention for treating a metalsludge obtained from a waste water sludge as a block diagram,

FIG. 3 shows a second treatment method according to the invention,

FIG. 4 shows a third treatment method according to the invention and

FIG. 5 shows a fourth treatment method according to the invention.

FIG. 1 shows diagrammatically processing of a sludge from a watertreatment plant. The metals in the sludge dissolve in the solutionduring acidification. The insoluble part i.e. the organic sludge isseparated, the organic sludge containing primarily fibres and possiblyinsoluble silicate minerals. For neutralizing the solution andprecipitation of metals e.g. lime is added to the solution. In theseparation stage subsequent to the neutralization stage, the metalsludge is separated. The filtrate is led to later stages of the wastewater treatment process.

According to FIG. 2 the metal sludge is dissolved in sulphuric acid orpossibly in hydrochloric acid in the dissolving stage and insolublesubstances are separated by filtering in the filtration stage.

The solution obtained from the filtration (filtrate) is led to theextraction stage. For example, following contents have been analyzedfrom the acidic solution: Fe (max. 6%) both ferric and ferrous,aluminium (max. 1%), Ca (max. 1000 ppm), Mg (max. 100 ppm), Pb (max. 100ppm), Cu, Ti, Cd, etc. The contents of metals in the solution may varyif Al has been used as the coagulant. The content varies also dependingon what neutralizing base has been used in the precipitation of theoriginal metal sludge. The extractant is a mixture of two or moreorganic compounds where the diluent for the effective agent like alkylphosphate is a straight-chain hydrocarbon like kerosene and, ifnecessary, an additional compound like a long straight-chain alcohol forenhancing the phases disengagement. Before the extraction stage thesolution can be treated with an oxidizing agent which oxidizes iron intoFe³⁺ and oxidizes organic substances into such form that they do nothinder extraction. Strong H₂ O₂ can be preferably used as the oxidizingagent. It is also possible that the oxidizing agent is added before thefiltration stage.

In the extraction stage the acid solution is contacted with the abovementioned organic solution whereupon trivalent iron and partly alsoaluminium are transferred from the acid solution into the organicsolution. Extraction is preferably performed at constant pH sinceextraction efficiency decreases with decreasing pH. pH can be adjustedby using alkali hydroxide or ammonia water, for example. Afterextraction, the organic phase and the aqueous phase are separated andthe obtained extract solution containing the iron and aluminium iscontacted with strong hydrochloric acid, for example, whereby trivalentiron and aluminium are transferred again to the acidic solution. Thisre-extraction or stripping can be also performed by using sulphuric acidwhereby trivalent iron is reduced to divalent form which has lesstendency to remain in the organic solution. The organic solvent, whichis free of metals, can be recycled back to the process. The acidicsolution obtained from the re-extraction i.e. the raffinate of strippingcontains iron and aluminium and can be further used for production ofcoagulants. Generally, the mixer/settler apparatus contains severalequilibrium stages with counter- or cross-flow of phases.

The acid solution (pH 0.5-1) obtained from the extraction stage or theraffinate of extraction contains mainly phosphorus but also someunextracted iron, aluminium and heavy metals. This raffinate must bepurified from organic residues e.g. by using active carbon before anyfurther treatment.

According to FIG. 2, sulphides like NaHS, Na₂ S, FeS or H₂ S are addedto the solution for precipitation of metals. Other known heavy metalbinders can also be used. The pH of the solution is adjusted to therange of about 3-4 to enable precipitation. NaOH can be used for the pHadjustment, for example. Since each heavy metal precipitates at aspecific pH, addition of sulphides is done simultaneously with raisingof pH from 3-4 to 6-7 to allow precipitation of all the heavy metalcomponents. The iron and aluminium which remain from the extractionstage precipitate as phosphates together with heavy metals whichprecipitate as sulphides, and/or hydroxides, and/or phosphates. Afterprecipitation, pH is raised to over 9, preferably to about 12, by addingNaOH so that the only soluble components which remain in the solutionare aluminium and phosphorus whereas other metals remain in insolubleform. In other words, the Al-phosphate which precipitated in the earlierstage dissolves as pH is raised.

In the filtration stage, which follows the precipitation of heavymetals, the precipitate is separated from the liquid phase containingthen primarily phosphate and sodium aluminate. The last residue of ironare separated at this stage along with the precipitate.

In the phosphorus precipitation stage, CaCl₂ is added to the filtratedsolution to precipitate phosphorus as calcium phosphate which isfiltrated in the subsequent filtration stage. The remaining filtratecontains soluble aluminium which is precipitated as aluminiumhydroxideby lowering the pH of the solution to level 7-8 using H₂ SO₄, forexample.

According to a second method, which is presented in FIG. 3,precipitation of heavy metals is performed at pH of about 5-8 using Na₂S and NH₄ OH for the pH adjustment. Heavy metals and unextracted Fe andAl precipitate as sulphides and phosphates. The precipitate is separatedafter which Mg is added as MgCl₂, MgO or Mg(OH)₂ and pH is raised ifnecessary to level 8-9 using NH₄ OH. Phosphorus in the solutionprecipitates as MgNH₄ PO₄ which can be used as a raw material forfertilizers. The amount Mg and NH₄ chemicals mentioned above isproportioned so that they are in stoichiometric ratio with the amount ofPO₄ to be precipitated. Preferably there is a small surplus of Mg.

According to another embodiment of the method of the invention heavymetal precipitation is performed at pH of about 4 using Na₂ S and NH₄ OHfor the pH adjustment. Heavy metals precipitate as sulphides and/orhydroxides and/or phosphates. Next, MgO is added to the solution and pHis raised to 5-6 whereby aluminium and chromium precipitate as phosphateand/or hydroxide. The precipitate is separated whereafter MgO and NH₄ OHare further added to the solution so that pH is about 9 wherebyphosphorus precipitates as MgNH₄ PO₄.

A possible procedure is that first the heavy metals are precipitated atpH 4 in the above described way whereafter aluminium and chromium areprecipitated at pH 5-6 and then filtration is performed.

After the extraction/stripping stage the raffinate solution alwayscontains some Fe and Al (see Table 2 in Example 5) which bind an equalnumber of moles of phosphorus. For example, the amount of Fe given inTable 2 in Example 5 binds an excessive amount of P of the raffinatesolution. Therefore, Fe and Al must be removed. If the pH of thesolution is raised FePO₄ and AlPO₄ will precipitate and the phosphorusis lost. There are several different ways to handle the problem ofremaining Fe and Al in the raffinate solution.

The first method is presented in FIG. 2. After the extraction/strippingstage, the remaining Fe and Al will be first precipitated as phosphatesalong with the heavy metals. Upon raising the pH with NaOH to about 12AlPO₄ will be redissolved. Ferric phosphate, on the other hand, willconvert to an insoluble ferric hydroxide, which is removed along withthe heavy metal precipitate, and the PO₄ ³⁻ of the ferric phosphate isdissolved.

The second method is presented in FIG. 3. In this case, the Fe and Alare precipitated together with the heavy metals as phosphates. Apartfrom losing the Fe and Al remaining in the raffinate solution, an equalmole number of P is also lost.

A third method for handling the remaining Fe and Al is shown in FIG. 4.In this case, there is an extra precipitation stage after theextraction/stripping. The pH is raised to about 3 with NaOH. The pH canvary in the range 2-4. At this pH, ferric phosphate, FePO₄, and possiblyAlPO₄ will precipitate at a good yield. The precipitate is thenseparated by filtration. The precipitate can be utilized as such or itcan be returned to the dissolution stage. The filtrate solution, free ofFe and Al, is treated in the same way as in the process of FIG. 3. Thedrawback of this method, in the case the FePO₄ precipitate is returnedto dissolution stage, is that the recycling leads to an increase in theP content of the extracting solution. This is not desired. It would bemore desirable to return only the Fe of the FePO₄ back to thedissolution stage and lead the PO₄ of the FePO₄ forwards in the process.

A fourth method is presented in FIG. 5. In this case the Fe and Al isprecipitated as FePO₄ and AlPO₄ which are separated by filtration. Next,the phosphate precipitate is dissolved with NaOH with a slight warming(60° C.) of the solution. Ferric phosphate is dissolved and solublesodium phosphate and insoluble Fe(OH)₃ is formed. Aluminium phosphate istotally dissolved. After separation, the ferric hydroxide precipitate isreturned to the beginning of the process and the sodium phosphatesolution, containing aluminium, is led to the subsequent process stagei.e. to the stage where heavy metals are precipitated or even to thestage of phosphate precipitation. Another possible process variant issuch that the sodium phosphate is precipitated from the solution and isutilized as such as a detergent phosphate.

Instead of returning the ferric hydroxide precipitate to the beginningof the process there is another interesting possibility. The ferrichydroxide let to dry and then it is added into the raffinate of thestripping stage i.e. to the aqueous phase. The added ferric hydroxidecontributes in two different ways: firstly it neutralizes the free acidof raffinate and secondly it raises the relatively low Fe-content of theraffinate solution.

It is worth noting in the process of FIG. 5 that the amount of NaOH usedfor dissolving the FePO₄ precipitate reduces the amount of NaOH neededat the precipitation of heavy metals. This means that no extra NaOH isneeded in the process of FIG. 5 compared to the process of FIG. 4 orFIG. 3. Furthermore, there is no increase in the P content of theextraction solution because phosphorus bound to Fe is not returned tothe beginning of the process but the soluble phosphorus is led forwardsin the process.

The process of the invention has several alternative modes of operation.One interesting alternative, which greatly improves the recovery ofnutrients i.e. phosphorus and ammonia of the waste water, is such thatthe metal sludge is formed by neutralizing the acidic hydrolysate with aMg-compound. This has the effect that MgNH₄ PO₄ 6H₂ O (struvite) willprecipitate into the metal sludge reducing the content of ammonia in thehydrolysate. If Mg-compounds and possibly ammonia are properly used inall pH adjustments of the process so that the number of moles of ammoniaequals to the number of moles of phosphorus corresponding to the moleratio in MgNH₄ PO₄, then after the final precipitation of MgNH₄ PO₄ noammonia will be left and none of it has to be returned to the wastewater process. Hence all nutrients have been recovered. If onlyMg-compounds have been used in pH adjustments the remaining filtratesolution contains primarily MgSO₄, which can be separated by evaporationand obtain a product of commercial value.

In the following, the invention will be further explained by means ofexamples.

EXAMPLE Example 1

A solution was obtained by leaching a metal containing waste watersludge with dilute H₂ SO₄ and filtering for gypsum removal atapproximately pH 1. The density of the solution was 1080 kg/m³ and itcontained 0.6% Fe²⁺, 1.8% Fe³⁺, 0.21% Al.

H₂ O₂ (10 ml) was added to the above solution (120 ml) during 60 minutesin a stirred cell. During feeding H₂ O₂ the temperature increased from20° C. to 55° C. The solution was filtered once more through a fuller'searth filter, the bed thickness of which was 10 mm. The filtratesolution (60 ml) was contacted with an organic extraction solvent (180ml) so that the phase ratio organic/aqueous=3/1. The organic solventconsisted of 22.5%MDEHPA, 67.5% kerosene and 10% 2-octanol. MDEHPAcontained 45% MEHPA and 55% DEHPA. After 20 minutes of mixing themixture was withdrawn to a separation funnel for phase separation. Theorganic phase was disengaged from the aqueous phase very rapidly, in10-15 seconds. Virtually no crud was detected between the phases. Theefficiency of the extraction is presented in Table 1a.

                  TABLE la                                                        ______________________________________                                        Relative amounts of metal components transferred                              from the aqueous phase to the extractant i.e. the efficiency                  of extraction (%)                                                                    Component                                                                             Efficiency                                                     ______________________________________                                               Total Fe                                                                              96.4%                                                                 Al      21.3%                                                          ______________________________________                                    

25 ml of the organic extract loaded with Fe and Al was contacted with 6MHCl in a stirred cell for 20 minutes. Thereafter the phases wereseparated in a separation funnel. The disengagement was again rapid andfollowing stripping efficiencies were obtained (Table 1b):

                  TABLE lb                                                        ______________________________________                                        Relative amounts of metal components transferred                              from the extractant to the stripping solution i.e. the                        efficiency of stripping (%)                                                          Component                                                                             Efficiency                                                     ______________________________________                                               Total Fe                                                                              52.9%                                                                 Al      70.8%                                                          ______________________________________                                    

Example 2

A solution from the extraction stage (207 g) contained 0.12% Fe, 0.18%Al, 1.62% PO₄ and heavy metals: 0.0018% Cu, 0.0009% Cr, 0.0124% Zn. 60 gof NaOH-solution (20%) was added to the solution and the pH rose to 2.7.Then 27.6 g of Na₂ S-solution (35%) was added whereby pH rose to 3.8. Aprecipitate was obtained which contained heavy metals as sulphides. Theprecipitate was filtered (2.5 g) and analyzed. A qualitative X-rayanalysis showed that the precipitate contained following heavy metals:Cu, Pb, As, V, Cr, Sr, Ni.

Next, a 57.6 g amount of NaOH (20%) was added to the filtrate (260 g)containing soluble aluminium and phosphorus, among other things,whereupon pH rose to the value of 13. It is typical of aluminium that aspH rises, aluminium first precipitates but begins to redissolve again aspH rises above 8. The aim of raising pH was to redissolve theprecipitated aluminium. In this stage, however, hydroxides precipitatedfrom the solution. The precipitate was filtrated and CaCl₂ -solution wasadded to the filtrate (285 g) and a calcium phosphate precipitate (31.8g) was obtained which, according to X-ray diffraction, contained Ca(OH)₂and Ca₅ (OH)(PO₄)₃. The precipitate contained 88% of the phosphorus ofthe Starting solution i.e the solution obtained from the extraction.Aluminium was also precipitated so that the precipitate contained 45% ofthe aluminium of the starting solution. The mole ratio PO₄ ³⁻ !/ Al! was5.0. In the last stage, H₂ SO₄ (10%) was added to the filtrate so thatpH sank to value 7. The purpose was to precipitate the aluminiumremaining in the solution. However, no precipitation occurred becausealuminium had already precipitated in the previous stages. The originalsludge was obtained by an iron coagulant so that amount of aluminium wassmall.

Example 3

In this example NH₄ OH was used for pH adjustment. Precipitation ofheavy metals was performed at pH 4.0 using Na₂ S-solution. The heavymetal precipitate contained 4% of the original phosphorus and 4% of theoriginal aluminium. Next, pH was raised to level 5.4. The precipitatecontained 24% of the original phosphorus and 63% of the originalaluminium. After precipitation, MgO and NH₄ OH were added to thesolution and pH rose to 9.0. A MgNH₄ PO₄ -precipitate (struvite) wasobtained with about 70% of the original phosphorus and about 30% of theoriginal aluminium. The mole ratio PO₄ ³⁻ !/ Al! was 6.0.

Example 4

In this example the procedure with respect to pH was the same as in theprevious example but now there was no filtration after the heavy metalprecipitation stage. Instead, the pH was raised to level 5.4 after theheavy metal precipitation. At this pH precipitation of phosphates tookplace. The precipitate contained 23% of the original phosphorus and 62%of the original aluminium. Adjustment of pH of the solution wasperformed using NH₄ OH. In this test the yield of phosphorus was of thesame order of magnitude as in example 3.

Example 5

The raffinate from the extraction/stripping stage had the analysis ofTable 2.

                  TABLE 2                                                         ______________________________________                                        Analysis of the raffinate.                                                           Element                                                                             Concentration                                                    ______________________________________                                               Fe    0.16%                                                                   Fe.sup.2+                                                                           0.11%                                                                   Al    0.12%                                                                   P     0.42%                                                                   Cu    6.9 ppm                                                                 Cr    5.6 ppm                                                                 Zn    7.3 ppm                                                          ______________________________________                                    

In order to further reduce the content of Fe in the raffinate, an extraprecipitation i.e. a polishing precipitation was performed in thefollowing way. The amount of the above raffinate used in theprecipitation was 817.5 g (750 ml) with a pH of 0.86. A 159.1 g amountof NaOH (30% was used to raise the pH of the raffinate solution to 2.8.After raising pH, a 1.5 g amount of H₂ O₂ was added to make sure thatall Fe was in trivalent form. The solution was then stirred for 1 h. Alight brown precipitate, identified as FePO₄ by X-ray methods, wasobtained. The solution was filtrated to separate the precipitate. Theamount the dry precipitate was 10.3 g. The precipitate was washed with83.9 g of distilled water. This washing water was added to the filtrateand the total amount of filtrate including the water was 984.6 g. Theanalysis of the filtrate obtained is given in Table 3.

                  TABLE 3                                                         ______________________________________                                        Analysis of the raffinate after removal of Fe by a                            polishing precipitation and the relative reduction of the                     elements.                                                                     Element     Concentration                                                                            Reduction-%                                            ______________________________________                                        Fe          0.005%     96.2                                                   Al          0.049%     50.8                                                   P            0.05%     85.7                                                   TOC          0.04%     --                                                     Cd          <0.3 ppm   --                                                     Cu          4.9 ppm    14.5                                                   Cr          3.8 ppm    18.3                                                   Ni          4.5 ppm    --                                                     Pb           <5 ppm    --                                                     Zn           57 ppm    6                                                      ______________________________________                                    

Results of this experiment indicate clearly that iron can be removedeffectively by an extra precipitation step. The obtained precipitate canbe either utilized as such as an additive in a fertilizer or it can bereturned to the beginning of the process i.e. to the dissolution stage.If it is returned to the dissolution stage, the phosphorus content ofthe process solution will gradually rise to an equilibrium value whichis the same the value given in Table 2.

Example 6

A 15 g amount of dried FePO₄ -cake (12% P, 24% Fe) precipitated from araffinate solution was dissolved into 56 g of water and 26 g of NaOH(50%). The pH was adjusted to 12 and the reaction vessel was slightlywarmed so that the temperature was 50° C. After 2 hours the precipitatewas filtrated and the precipitate was washed with water. The amount ofFe(OH)₃ precipitate obtained was 7.2 g (dry weight) containing 50% Feand 2.2% P. This result indicates that an almost complete conversion ofFePO₄ to Fe(OH)₃ has taken place in terms of the iron content.Therefore, all Fe of the raffinate can be removed and returned to thebeginning of the process. The filtrate contained less than 0.002% Fe.The yield of P in the filtrate was more than 90%. This portion of the Pcan be separated to be led to the following process stage.

Example 7

A raffinate solution after separation of FePO₄ and returning of P hadthe analysis of Table 4 (column A). To precipitate heavy metals fromthis solution, NaHS and NaOH were added. The precipitate was filtratedand washed with water. The filtrate solution was analyzed for the heavymetals and the results are given in Table 5 (column A). Since theconcentrations of the original raffinate were so low, below thedetection limit of the elements, these results were not very reliable inshowing the efficiency of sulphide precipitation in removing the heavymetals. Therefore, another test was made with the same raffinate butwith added heavy metals. The heavy metal concentrations of theraffinate, after the addition, are given in Table 4 (column B).

                  TABLE 4                                                         ______________________________________                                        Analysis of the raffinate after removal of Fe and                             return of P. A = original raffinate, B = after addition of                    heavy metals.                                                                             Concentration                                                                            Concentration                                          Element     A          B                                                      ______________________________________                                        Fe          0.005%                                                            Al          0.049%                                                            P            0.39%                                                            TOC          0.04%                                                            Cd          <0.3 ppm     20 ppm                                               Cu          4.9 ppm    24.9 ppm                                               Cr          3.8 ppm    23.8 ppm                                               Ni          4.5 ppm    24.5 ppm                                               Pb           <5 ppm      25 ppm                                               Zn           57 ppm     117 ppm                                               ______________________________________                                    

To an amount of 220.6 g of the above solution, 370 mg NaHS (40%) and 9.0g NaOH (30%) were added to precipitate heavy metals as sulphides. The pHrose to 8.5. The precipitates were filtrated and washed with 51.2 g ofwater. The dry weight of the heavy metal cake was 1.725 g. The amount offiltrate solution (including the wash water) 261 g. The heavy metalconcentrations of the filtrate and the corresponding reductionpercentages are given in Table 5 (column B).

                  TABLE 5                                                         ______________________________________                                        Analysis of the raffinate solution after                                      precipitation of heavy metals, the right column gives the                     relative reduction of the concentration of the elements.                      Ele-   Concentration  Concentration                                                                            Reduction                                    ment   A              B          B                                            ______________________________________                                        P      0.27%          --         --                                           Mg     0.036%         --         --                                           NH4    0.04%          --         --                                           Al     <0.004%        --         --                                           Cd     <0.3 ppm       <1.2 ppm   >94%                                         Cu     <2 ppm         <2.4 ppm   >90%                                         Cr     <3 ppm         <3.6 ppm   >85%                                         Ni     <2 ppm         <2.6 ppm   >91%                                         Pb     <5 ppm           <5 ppm   >76%                                         Zn     0.3 ppm        0.38       >99%                                         ______________________________________                                    

Results of Table 5 shows that heavy metals can be effectively removedfrom the raffinate solution by sulphide precipitation. There is someloss in phosphorus. This is partly due to aluminium which bindsphosphorus at this pH and partly formation of magnesium phosphate.

Example 8

Table 6 presents results from tests for precipitating heavy metals atvarious pH. The differences in the results are small. If pH goes over 6there is a danger that MgNH₄ PO₄ starts to precipitate leading tophosphorus losses in the raffinate.

                  TABLE 6                                                         ______________________________________                                        Analysis of the original raffinate solution before                            precipitation of heavy metals and after precipitation at                      various pH.                                                                                     Filtrate after precipitation                                Element   Original                                                                              pH 5.1      pH 6.0                                                                              pH 6.6                                    ______________________________________                                        Cr (ppm)  4.2     <2          <2    <2                                        Ni (ppm)  5.4     2.4         <2    <2                                        Zn (ppm)  60      <0.3        <0.3  <0.3                                      P (%)     --      0.35        0.36  0.34                                      ______________________________________                                    

Example 9

To an amount of 250 g of the filtrate solution from example 7, 5.32 g ofMgCl₂ and 4 g of NH₃ -solution (25%) were added to precipitate P asMgNH₄ PO₄. The pH was adjusted to 9. The solution was filtrated yieldinga cake with a dry weight of 3.55 g. The cake was analyzed by XRD and wasidentified as MgNH₄ PO₄ 6H₂ O (struvite). The cake was dried to a dryweight of 3.55 g and analyzed. The dried cake contained 14% Mg, 4.5%NH₄, and 20.8% P. The crystal water and half of the ammonia hadevaporated during drying due to a too high temperature. The yield of Pin this precipitation was 96.3%. The filtrate solution (246.5 g)contained <0.01% P.

Example 10

A 118 g amount of the filtrate, from which heavy metals had been removedby sulphide precipitation and which contained 0.23% P, was taken and pHwas adjusted from 8.0 to 6.0 with 96% H₂ SO₄. A 1.93 g amount of CaCl₂2.H₂ O was then added and the batch was mixed for 2 hours, thetemperature being about 50° C. The pH of the batch was then raised from5.3 to 8.5 by adding 0.63 g Ca(OH)₂. After mixing for 2 h, the batch wasfiltrated yielding a cake with a dry weight of 2.55 g and a 106.5 amountof filtrate. The cake was analyzed by XRD and was identified as Ca₃(OH)(PO₄)₃. It contained 12% P and 29% Ca. The filtrate contained <0.01%P.

Example 11

A metal sludge with DS of 29.6% was calcined at 325° C. for 1 h. Theignition loss was 26.8%. The calcinate containing 17% Fe, 3.2% P, 5.5%Al was then dissolved in sulphuric acid (96%) for 2 h at a temperatureof over 100° C. The solution was filtrated and the filtrate contained4.6% Fe,0.38% Al, 0.86% P, and 0.05% TOC. In the extraction stage therewas no crud formation and the extraction rate was high. Extractionefficiency was of the same order as in parallel extraction tests withthe hydrogen peroxide pre-treatment without any calcination.

The invention is not limited to the above examples but can be variedwithin the limits of the enclosed claims. Therefore, dissolution ofmetals from waste water sludge can be performed without thermaltreatment. Thermal treatment is advantageous since it improvesfilterability of the organic sludge and solvability of metals. It isessential for the implementation of the method of the invention that themetals in the waste water sludge are first dissolved throughacidification and then precipitated as phosphates and hydroxides.

We claim:
 1. A method for treating a waste water sludge, the sludgecomprising phosphorous, heavy metals and at least one metal originatingfrom a waste water treatment coagulant, in order to recover thephosphorous and the at least one metal, and to discharge the heavymetals, wherein the at least one metal comprises iron or both iron andaluminum, the method comprising:acidifying the sludge to dissolve metalscontained therein; subjecting the acidified sludge to liquid-liquidextraction to thereby provide an aqueous phase or raffinate, andrecovering at least a portion of the at least one metal; treating theraffinate, during a first precipitation stage, with an effective amountof chemicals to provide a precipitate comprising heavy metals, and asolution having a decreased content of heavy metals; discharging theprecipitate comprising heavy metals; treating the solution having adecreased content of heavy metals, in a second precipitation stage, withan effective amount of chemicals to provide a precipitate comprisingphosphorous; and recovering phosphorous from the precipitate comprisingphosphorous.
 2. The method of claim 1 further comprising treating thesludge, after the acidifying and prior to the liquid-liquid extraction,with an oxidizer in order to convert organic material contained in thesludge into a form that does not have an unfavorable effect on theliquid-liquid extraction, and to oxidize divalent iron contained in thesludge into trivalent iron.
 3. The method of claim 2 wherein theoxidizer is hydrogen peroxide.
 4. The method of claim 1 furthercomprising calcination of the sludge, after the acidifying, in order toincinerate organic material contained therein.
 5. The method of claim 1wherein the liquid-liquid extraction comprises an extraction stage and astripping stage; the method further comprisingcontacting the sludge,during the extraction stage, with a water-immiscible extraction solutionto thereby form both a first aqueous phase or the raffinate, and a firstorganic phase, wherein the first organic phase is loaded with ions ofthe at least one metal; separating the raffinate from the first organicphase; contacting the first organic phase, during the stripping stage,with an acidic aqueous stripping solution to thereby form both a secondorganic phase and a second aqueous phase, wherein the second aqueousphase is loaded with ions of the at least one metal; and separating thesecond organic phase from the second aqueous phase.
 6. The method ofclaim 5 wherein the extraction solution comprises an organic phosphateand an organic solvent.
 7. The method of claim 6 wherein the organicphosphate is an alkyl phosphate selected from the group consisting ofmonoalkyl phosphate, dialkyl phosphate, trialkyl phosphate and mixturesthereof.
 8. The method of claim 7 wherein the alkyl phosphate isselected from the group consisting of mono(2-ethylhexyl)phosphate,di(2-ethyl)phosphate, and tributyl phosphate.
 9. The method of claim 6wherein the organic solvent comprises a long-chained hydrocarbon. 10.The method of claim 9 wherein the long-chained hydrocarbon compriseskerosene.
 11. The method of claim 6 wherein the organic solventcomprises a long-chained alcohol.
 12. The method of claim 11 wherein thelong-chained alcohol is 2-octanol.
 13. The method of claim 1 furthercomprising,treating the raffinate, prior to the first precipitationstage, with an alkali hydroxide to thereby provide a precipitatecomprising iron phosphate; separating the precipitate comprising ironphosphate.
 14. The method of claim 13 wherein the precipitate comprisingiron phosphate further comprises aluminum phosphate.
 15. The method ofclaim 13 wherein the alkali hydroxide is added in an amount effective toraise the pH of the raftmate to a value between 2 and 4, and the alkalihydroxide is sodium hydroxide.
 16. The method of claim 13 wherein theprecipitate comprising iron phosphate is treated with an alkalihydroxide to thereby form insoluble iron hydroxide and a solutioncomprising soluble alkali phosphate.
 17. The method of claim 16 whereinaluminum hydroxide is formed.
 18. The method of claim 16 wherein thesolution comprising alkali phosphate is introduced into the first orsecond precipitation stages.
 19. The method of claim 1 or 13 furthercomprising,treating the raffinate, in the first precipitation stage,with a heavy metal binder at a pH effective to provide the precipitatecomprising heavy metals; separating the precipitate comprising heavymetals; and treating, in the second precipitation stage, the solutionhaving a decreased content of heavy metals with aphosphorous-precipitating reagent at a pH effective to precipitatephosphorous in a form comprising phosphate.
 20. The method of claim 19wherein the heavy metal binder is a sulfide selected from the groupconsisting of hydrogen sulfide, sodium sulfide, sodium hydrogen sulfideand ferrous sulfide.
 21. The method of claim 19 wherein the pH effectiveto provide the precipitate comprising heavy metals is also effective toprovide a precipitate comprising iron.
 22. The method of claim 19wherein the pH effective to provide the precipitate comprising heavymetals is also effective to provide a precipitate comprising aluminum.23. The method of claim 19 wherein the phosphorous-precipitating reagentis selected from the group consisting of calcium oxide, calciumhydroxide, magnesium oxide, magnesium hydroxide and magnesium chloride.24. The method of claim 19 wherein the solution having a decreasedcontent of heavy metals is treated to provide a precipitate comprisingcalcium phosphate or magnesium phosphate.
 25. The method of claim 19further comprisingtreating the raffinate, in the first precipitationstage, with sodium sulfide, and adjusting the pH of the raffinate to avalue between 5 and 8, to provide a first precipitate comprising heavymetals and an aqueous solution; separating the first precipitate;treating the aqueous solution, in the second precipitation stage, with acalcium or magnesium compound to provide a second precipitate; andseparating the second precipitate.
 26. The method of claim 25 whereinthe pH of the raffinate is adjusted to a value between 5 and 8 with abase selected from the group consisting of sodium hydroxide, magnesiumoxide, magnesium hydroxide and ammonia.
 27. The method of claim 25wherein iron is precipitated in the first precipitation stage.
 28. Themethod of claim 25 wherein aluminum is precipitated in the firstprecipitation stage.
 29. The method of claim 25 wherein the calcium ormagnesium compound of the second precipitation stage is selected fromthe group consisting of calcium chloride, magnesium oxide, magnesiumhydroxide and magnesium chloride.
 30. The method of claim 25 wherein, inthe second precipitation stage, the pH of the obtained aqueous solutionis adjusted to provide for precipitation of a phosphate selected frommagnesium phosphate and calcium phosphate.
 31. The method of claim 19further comprising,simultaneously treating the raffinate, in the firstprecipitation stage, with a sulfide selected from the group consistingof hydrogen sulfide, sodium sulfide, sodium hydrogen sulfide and ferroussulfide, and a base sufficient to raise the pH of the raffinate tobetween 6 and 7; combining the simultaneously treated raffinate withadditional base to raise the pH to greater than 9 and re-dissolvealuminum precipitates, if present; separating first precipitatecontaining heavy metals and obtaining an aqueous solution comprisingphosphorous and aluminum; treating the aqueous solution comprisingphosphorous and aluminum, in the second precipitation stage, withcalcium chloride to provide a precipitate comprising calcium phosphateand an aqueous solution comprising aluminum; separating the precipitatecomprising calcium phosphate; treating the aqueous solution comprisingaluminum, in a third precipitation stage, with an acidic compound toprovide a precipitate comprising aluminum hydroxide; and separating theprecipitate comprising aluminum hydroxide.
 32. The method of claim 31wherein the acidic compound is sulfuric acid.
 33. The method of claim 31wherein the additional base raises the pH to about
 12. 34. The method ofclaim 31 wherein the base sufficient to raise the pH of the raftmate tobetween 6 and 7 is sodium hydroxide.
 35. The method of claim 19 furthercomprising,treating the raffinate, in the first precipitation stage,with sodium sulfide and adjusting the pH to a value of between 3 and 4using sodium hydroxide, to provide a first precipitate comprising heavymetals and an obtained aqueous solution; separating the firstprecipitate; adjusting the pH of the obtained aqueous solution to avalue of about 12 or greater using sodium hydroxide, to provide a secondprecipitate and an aqueous solution comprising aluminum and phosphorous;separating the second precipitate; treating the aqueous solutioncomprising aluminum and phosphorous, in the second precipitation stage,with calcium chloride, to provide a third precipitate comprising calciumphosphate and an aqueous solution comprising aluminum; separating thethird precipitate; treating the aqueous solution comprising aluminum, ina third precipitation stage, with an acidic compound, to provide afourth precipitate comprising aluminum hydroxide; and separating thefourth precipitate.
 36. The method of claim 1 further comprising a thirdprecipitation stage wherein aluminum is precipitated as aluminumhydroxide.
 37. The method of claim 1 wherein the sludge comprises metalsludge obtained by subjecting waste water sludge from a waste watertreatment plant to acid treatment followed by precipitation of the metalsludge from a filtrate.